The transition metals include the nine metals of Group VIII of the periodic table, as representative examples thereof, and the lanthanide and actinide metals. The transition metals further include the metals called rare metals, noble metals, and heavy metals. Specific examples thereof include iron (Fe), cobalt (Co), nickel (Ni), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), platinum (Pt), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), copper (Cu), zinc (Zn), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), silver (Ag), cadmium (Cd), lanthanum (La), cerium (Ca), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), hafnium (Hf), tantalum (Ta), tungsten (W), gold (Au), mercury (Hg), uranium (U), and plutonium (Pu). These transition metals are used not only in catalysts and iron/steels but also in a wide variety of other applications such as hydrogen-absorbing alloys, batteries, magnets, and superconductive materials. These metals are desired to be recovered from so-called secondary resources from the standpoint of stable supply. Furthermore, it in necessary to treat industrial drainage and the like to remove the metals contained therein in slight amounts. Thus, the establishment of an efficient metal recovery technique is an important subject also from the standpoint of environmental protection.
One of the elemental techniques for the recovery and purification of transition metals is the solvent extraction method.
The solvent extraction method has conventionally employed an acid, basic, or neutral extracting agent according to the composition of the solution to be treated. Examples of the extracting agent include di(2-ethylhexyl)phosphoric acid, E-2-hydroxy-5-nonylbenzophenone oxime, 8-hydroxy-7-(1-nonyl-2-propenyl)quinoline, trialkylmethylammonium salts, and tributyl phosphate. The extracting agents are commercially available, for example, under the trade names of D2EHPA (Kao Corp., Daihachi Chemical Industry Co., Ltd.), Lix 65N (General Mills), Kelex 120 (Ashland Chemical), and Aliquat 336 (Ashland Chemical).
Besides being used alone, transition metals have recently come to be used as composites and similar materials such as alloys and mixtures. It is hence thought that the solvent extraction method comes to be utilized increasingly. There is a desire for an extracting agent which has higher extraction capacity, higher extraction rate, and higher selectivity and is harmless and inexpensive.